1. Field of the Invention
The present invention relates to a piezoelectric micro-blower suitable for conveying compressible fluid such as air and gas.
2. Description of the Related Art
A piezoelectric micro-blower is known as an air blower for dissipating heat generated in a housing of a portable electronic apparatus or for supplying oxygen required to generate electric power in a fuel cell. The piezoelectric micro-blower is a type of pump which includes a diaphragm which bends when a voltage is applied to a piezoelectric element, and is advantageous in that the piezoelectric micro-blower can be configured to have a simple structure, small size and thickness, and low power consumption.
Japanese Unexamined Patent Application Publication No. 64-2793 (FIG. 14) discloses a flow generating apparatus including a piezoelectric element. In the flow generating apparatus, as shown in FIG. 14, a compression chamber 103 is formed between a base 100 and a nozzle plate 101, a ring-shaped piezoelectric element 104 is fixed to the nozzle plate 101, and a plurality of nozzle holes 102 is formed in the central portion of the nozzle plate 101. A case 105 is provided so as to surround the base 100 at a predetermined interval, and a cylindrical guide 106 is formed at a portion of the case 105 which faces the nozzle holes 102. By driving the piezoelectric element 104 at a high frequency, the nozzle plate 101 is flexurally vibrated, a jet flow is generated from the plurality of nozzle holes 102, and the airflow discharged from the nozzle holes 102 can be discharged from the guide 106 of the case 105 to the outside while drawing the ambient air.
In Japanese Unexamined Patent Application Publication No. 64-2793, by driving the piezoelectric element 104, the central portion of the nozzle plate 101 greatly flexurally vibrates and a jet flow can be generated in accordance with the displacement of the nozzle plate 101. However, the wall portion of the base 100 which faces the nozzle plate 101 across the compression chamber 103 is a fixed wall, and thus, a significant increase in flow rate cannot be expected only by the vibrations of the nozzle plate 101.
Japanese Unexamined Patent Application Publication No. 2006-522896 discloses a gas flow generator. As shown in FIG. 15, the gas flow generator includes an ultrasonic driver 110 in which a ring-shaped piezoelectric element 112 is fixed on a ring-shaped base 111, a first stainless-steel membrane 113 fixed to a lower surface of the driver 110, a second stainless-steel membrane 114 mounted parallel to and at a predetermined interval from the first membrane 113, and a spacer 116 retaining the membranes 113 and 114 such that the membranes 113 and 114 are spaced apart from each other. The central portion of the first membrane 113 bulges downwardly, and the second membrane 114 has a plurality of holes 115 formed in the central portion thereof.
In the case of the gas flow generator, when the ultrasonic driver 110 is driven at a high frequency, air is discharged in the orthogonal direction of the holes 115 while the air around the holes 115 formed in the central portion of the second membrane 114 is sucked or drawn, whereby an inertial jet can be generated. However, the space around the holes 115 in the second membrane 114 is an opened space, and thus the discharged airflow diffuses and a desired flow rate cannot be obtained. In addition, a vortex of air occurs around the holes 115 and great noise occurs.
Thus, the applicant of the present application has proposed a piezoelectric micro-blower having high pressure and flow rate (International Publication No. WO2008/69266). As shown in FIG. 16, the micro-blower includes a blower body 120, a vibrating plate 121 which is fixed at an outer peripheral portion thereof to the blower body 120 and includes a piezoelectric element 122, and a blower chamber 123 formed between the blower body 120 and the vibrating plate 121. A first wall portion 124 is provided at a location facing the vibrating plate 121 across the blower chamber 123 and resonates with vibrations of the vibrating plate 121. The first wall portion 124 has a first opening portion 125 formed in the central portion thereof. A second wall portion 126 is provided on the opposite side of the first wall portion 124 with respect to the blower chamber 123. The second wall portion 126 has a second opening portion 127 formed in a portion thereof facing the first opening portion 125. An inflow passage 129 is formed between the first wall portion 124 and the second wall portion 126 and communicates with inlets 128. When the vibrating plate 121 vibrates, fluid is ejected from the first opening portion 125 due to a change in volume of the blower chamber 123, and can be discharged from the second opening 127 to the outside while drawing the ambient fluid in the inflow passage 129.
In the piezoelectric micro-blower, when the vibrating plate 121 is vibrated, fluid is sucked through the first opening 125 in a first half cycle and then is discharged in the next half cycle. However, because the fluid is discharged from the second opening 127 while the ambient air is drawn by a high-speed airflow discharged from the first opening 125, a discharge flow rate larger than the displaced volume of the vibrating plate 121 can be obtained at the second opening 127. In addition, when the first wall portion 124 is resonated with vibrations of the vibrating plate 121, the displaced volume of the vibrating plate 121 is increased by displacement of the first wall portion 124, whereby high pressure and flow rate can be obtained. Such a superior effect is provided but great noise (e.g., wind noise) occurs near the first opening 125.